THALLOPHYTA 35 



SUB-DIVISION D, ALG^ 



The seaweeds are primitive, water-dwelling plants, ranging 

 from microscopic, one-celled forms to large and complex plants. 

 They show an advance over some other groups of the thallo- 

 phytes in the presence of the green coloring matter, — chloro- 

 phyl, which is, however, in certain of the algae masked by brown 

 or red pigments. There is great variety in the method of 

 reproduction. It may be either asexual through the production 

 of special cells, — the spores, which later develop into new 

 plants, or sexual through union of male and female cells. Upon 

 other structural and reproductive characters coupled with the 

 difference in color is based the classification into green, brown, 

 and red algae. Algae have been recognized from the pre-Cam- 

 brian to the present. 



I. Green algae. — Common examples are: — 



a. Spirogyra, the common green alga floating on ponds where 

 its long green threads form the frothy masses called pond scum. 



h. Halimeda, a lime-secreting alga of modern seas. It grows 

 on coral reefs, connecting dead coral masses. It forms exten- 

 sive lim.estone deposits in the lagoons of coral atolls, as in the 

 Funafuti reef. The analysis of the plant, which gives over 

 90 per cent of lime (CaCOs) and only 3 jq per cent of organic 

 matter, shows its importance as a rock builder. 



c. Chara (stonewort). (There is a growing tendency to re- 

 move Chara from the green algae and to raise it into a distinct 

 group.) This plant inhabits fresh w^ater lakes of the temperate 

 region. It secretes lime with w^hich it encrusts its leaves and 

 stems, making them white and brittle. Upon the death of the 

 protoplasmic part of the plant, this lime deposit disintegrates 

 and settles as a Hmy mud, the marl of marl ponds. Chara is 

 responsible for many fresh water limestones of the past. Char- 

 acteristic spore cases occur in the mid-Devonian at the Falls of 

 the Ohio and in the Hamilton of Missouri, and it is well known 

 since the Upper Jurassic. It was especially abundant in the 



